The present invention relates to an apparatus for scanning a surface with electromagnetic radiation, such as a printer or a scanner.
The scanning of a surface with electromagnetic radiation is used in many applications.
In particular, many printers use an electromagnetic beam to expose a printing medium having a printing surface that is sensitive to the electromagnetic radiation of the beam. In order to expose the printing material according to predetermined digital input data, the electromagnetic beam is typically scanned across the printing surface and modulated according to the digital data. When the printing surface is a flat surface, such a printer is generally referred to as a flatbed printer.
Furthermore, optical scanners utilize electromagnetic radiation to obtain digital information from an original, e.g., electronic document, by scanning the original in longitudinally successive, transversally extending sections by means of a semiconductor camera or other radiation detector.
In such applications, it is generally desirable to maintain a constant distance between the optical system used in the scanning process and the surface to be scanned. The unit comprising the optical system will be referred to as the optical head or—in case of a printer—the print head.
U.S. Pat. No. 4,941,065 discloses a scanning apparatus where a semiconductor camera is guided across the original, or across a glass plate covering the original, in a transversely arranged, longitudinally displaceable portal-shaped device.
According to the above prior art apparatus, the distance between the original—or the glass plate covering the original—and the camera is maintained by the camera being moved directly across the original or the glass plate, respectively. This allows an exact adjustment of the distance between the camera and the original to be made, although in the latter instance, the intermediate glass plate is not quite planar. In order to avoid scratching the surface of the glass plate, the distance between the camera and the original or the glass plate covering the original is maintained by means of a fluid film generation unit that causes the camera unit to “float” on a film of pressurized air. The fluid film generation unit comprises a ring-shaped member with downwardly directed openings through which pressurized air is provided. The camera is positioned such that it receives light through the central aperture of the ring. This allows an adjustment of the distance from the camera to the upper surface of the original or the glass plate (whichever is uppermost), to be made without the semiconductor camera physically contacting the original or the glass plate, respectively.
However, in particular in printing applications, it is often not desirable to cover the printing medium with a protective glass plate. In many applications, the scanning beam is a light beam in the ultra-violet (UV) region which would be distorted by the glass plate. Hence, it is desirable to scan with a print head unit directly across the printing plate placed on a base plate. On the other hand, a scanning with a UV beam generally requires a particularly accurate control of the distance between the printing surface and the print head, because in such systems the aperture of the optical system is typically large, resulting in a small depth of focus.
Furthermore, it is often desirable to print on the entire area of a printing plate, in particular without leaving an unexposed margin at the edges of a printing plate. However, at the edges of the printing surface, the fluid film of the above prior art system is disturbed due to the height difference between the upper surface of the printing plate and the upper surface of the underlying base plate on which the printing plate is placed. Consequently, it is a problem of the above prior art apparatus to accurately control the distance between the optical head and the surface of the medium to be scanned even at the edges of the medium.